The beam corresponding to such illumination is a short range beam which is limited in the upwards direction by a substantially horizontal cutoff plane, which is spread very wide in the sideways direction, and which does not include any rising rays which could give rise to undesirable optical phenomena in conjunction with the droplets of water in suspension in a fog.
Prior art foglights generally include an axially oriented filament, i.e. a filament lying on the axis of the foglight.
Several solutions have been proposed for the associated reflector. The first solution consists in providing a reflector in the form of a paraboloid of revolution whose focus is situated at the front end of the filament (in the light emission direction). This provides a slightly divergent beam and the rays in its top half are deflected downwardly by deflector elements provided in the closure glass.
However, this gives rise to considerable excess thickness in the glass which makes it difficult to mold, particularly when the glass is literally made of glass.
Proposals have also been made to form a reflector using two axially offset half-paraboloids. The top half-paraboloid is focused on the rear end of the filaments so as to form a conventional cutoff beam while the bottom half-paraboloid is focused on the front end of the filament so that all the images of the filament are situated beneath the cutoff.
Such a foglight suffers from two major drawbacks. Firstly the reflector has a surface discontinuity where the two half-parabolids meet: since the paraboloids are focused on different points, they necessarily have different apexes, or else they have different focal lengths, and in either case they have different profiles along a connection plane; as a result of this, a reflector manufactured in accordance with this teaching is never perfect, in practice, at the connection, thereby giving rise to parasitic light rays being emitted above the cutoff.
Secondly, and above all, the beam generated by the bottom half-paraboloid is spread out sideways in a satisfactory manner only in a zone situated immediately below the cutoff. This non-uniform spread of the beam goes against the desired aim of a fog lamp beam which is to obtain relatively uniform sideways spread.
Finally, proposals have been made in French published patent application No. 2 536 503 in the name of the Applicant, for a foglight in which the filament remains axially oriented and the reflector comprises a composite surface without discontinuity suitable for forming images of the filament in which nearly all the points of the images are situated beneath the cutoff. Such a solution makes it possible to obtain a completely uniform sideways spread of the beam together with a well-defined cutoff.
However, using an axial filament with a reflector of this type necessary gives rise to filament images which are large in size and which are vertically oriented or at a small angle to the vertical. Such images need spreading sideways by a considerable amount, and to do this it is necessary to provide appropriate prisms or ribs on the closure glass. A second drawback of this type of fog lamp, likewise due to the existence of such vertical images, is that the beam is thicker than required in practice, with the bottom portion of the beam not contributing to useful illumination and possibly even constituting a hinderance by illuminating the road too close to the vehicle.
Finally, prior art foglights exist which include a lamp having a horizontal filament extending across the foglight axis, and associated with a reflector in the form of a paraboloid of revolution whose focus is situated at the center of the filament. In this way, the beam does not include large filament images, however it remains necessary to deflect the images in a vertical direction using prisms formed in the closure glass and with the consequent risk of emitting parasitic rays in an upwards direction from the edges of the prisms.
The present invention seeks to mitigate the drawbacks of the prior art and to provide a foglight in which the closure glass needs to provide substantially no vertical deflection of the light rays, and in which the beam obtained is properly uniform in width as well as in height.